06 January 2013

Shooting in the wind

Wind calls are part science and art. Science because we can accurately predict what a bullet will do in a given set of conditions.  Art because in reality, wind behaves differently at different ranges.  I was shooting at a 500 yard High Power match where the flags at 500 stood straight to the left, the flag at 300 straight to the right, and the flag on the target line hung limp.  Safe to say that my wind reading skills aren't that good, and more than one Master ranked shooter that day said, "screw it" and stopped fiddling with sights and started using holdovers as the conditions changed randomly.

I have tried different software solutions over a decade ago to help train wind calls, and I'm sure that they are helpful to some people.  I found them frustrating and not very helpful, but I should probably go back and see if there are any current software solutions that will be helpful for training wind calls instead of just maintaining skills.  The first part of reading the wind is learning how to estimate wind speed based on how it interacts with flags, trees, grass, mirage, etc.

Wind Rosette chart from Flea, snipersparadise.com
The second part is is figuring out what the wind is doing between you and the target.  The wind conditions from halfway between you and the target to the target get more time to influence the bullet, so the accepted wisdom is to use that wind condition to set up your shot. 

There are two common ways to predict wind drift.  The first is to do the equation, d = w (t-t_0) which is not helpful.  Deflection equals wind velocity multiplied by the quantity of time of flight actual minus time of flight in a vacuum (ie, constant muzzle velocity).  So there is another equation that will "get you close" which is; Range [hundreds] x wind speed [mph] / constant = MOA adjustment.  The problem with this formula is that the "constant" isn't a constant, it is actually a non-linear function in its own right.  For 200 meters the constant is 10, for 800 meters the constant is 7, with an M855 bullet at 3100 fps.  When going through the SDM train the trainer course, we were told, "use 10, it will get you close enough."  Obviously this is just a simplification to get a tactical shooter on a torso sized target.

I'm a math geek, and I don't like these formulas because they are difficult to do in your head when you are behind the rifle trying to keep your front sight or cross hairs on target (red dot aiming points need not apply).

So there is the second method, which is a bit simpler.  Print out a dang ballistics table for the conditions you are shooting in.

I used the JBM Ballistic trajectory calulator (simplified) to generate this chart.  I set the zero range for 300 meters, and set the interval range for 50 meters.  This is more than enough data for an SDM to use.

The wind data is set at 10 mph, and this is a good baseline.  Got a 3 mph wind?  Divide your number by 4 to get a 2.5 mph wind correction and use that.  Got a 5 mph wind? divide by two and use that.  See where I'm going?  with simple mental math you can get pretty close, really quickly.

Now, if you don't want to do any mental math, or pull out your handy dandy calculator from your final firing position, you can do the math ahead of time to get a wind drift chart that is specific to your altitude, bullet choice, and muzzle velocity.

Wind and Lead chart, courtesy of Flea, www.snipersparadise.com
If you don't have a spotter, this is the only way to go that can get you on target relatively quickly. 

The chart to the left was generated using the same JBM ballistic calculator, but setting the wind speed to 1 mph (and this is using a standard M118LR load, not the M855 chart above).

If I do the same exercise for M855 at 3100 fps, and extend the columns for more than just 1 mph and multiply by the wind speed, I end up with something such as this.  No math necessary, print and laminate, keep handy for when you are on the firing point and have done your wind speed and direction estimate.  With a service rifle using stock sights, anything under 0.5 MOA is too small to account for (the sights are pretty rough).  With competition irons, 0.25 MOA is the limit of precision.  I don't know anyone who competes with a 0.125 MOA (1/8th MOA) scope, but they make them so I'm sure someone has them.

The chart above is for standard temp and atmospheric pressure.  Obviously a change in atmospheric density (due to temperature, humidity or altitude changes means you need to make a chart for those conditions).

If you don't making 10 ballistic charts each with the exact windspeed from 1 to 10, you don't even have to do math, just type in the numbers on your spreadsheet.  This is a "full value" chart (wind at 9 or 3), and it is easy to make a 0.866 and 0.707 chart simply by multiplying each value by .866 or .707 and labeling your chart as such.

So there it is, the easiest way to do the math for a wind correction is now, on your computer.  Print out the 3 wind tables you need for your cartridge (full value, .866 value, .707 value, you could do a half value if you really wanted to).  I guarantee you that you can flip through 4 laminated cards faster than you can plug an equation into a calculator.

What a wind chart can't give you is experience.  And if you want to get experience, there are no shortcuts.  If you don't shoot prairie dogs, compete in High Power or F-Class (or Palma), you are probably not building the experience necessary for being a good shot in the wind.


Anonymous said...

If my memory serves me correctly from a two decades old copy of the Army sniper FM the "constant" formula was the method taught. RRS

AM said...

The constant formula is the method taught. The constant that was taught at the sniper school was "15" if my memory is correct. Different bullet at a different velocity, but still only an approximation of a non-linear function.

Of course for shallow curves a linear approximation is good enough quite a bit of the time.

StukaPilot said...

Yeah,,,ahhh...hmmm. Believe I'll just get off three quick shots. One a bit to right of target, one a bit to left, one straight at. Or wait 'til the opposition is really, really close. Or avoid windy combat zones altogether. Or not fight on windy days.

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